Optical-fiber Bragg gratings (FBGs) have become essential components in the telecommunications industry, where they perform various spectral filtering operations. A fiber Bragg grating consists of a periodic modulation of the index of refraction along the core of an optical fiber. It is created by exposing a photosensitive fiber to a properly shaped intensity pattern of ultraviolet light. This light produces a permanent change in the index of refraction in selected sections of the optical fiber. The resulting optical fiber grating behaves as a wavelength-selective reflector having a characteristic spectral response. In the case of a WDM filter, the reflected wavelength of light is often referred to as the grating wavelength or as the Bragg wavelength of the grating. Their stability and reliability, in conjunction with their all-guided-wave nature, have made FBGs ideal candidates for fiber optic system applications. They now play an important role in numerous optical fiber devices for guided-wave optics and fiberoptic-system applications.
Fiber Bragg gratings are now used extensively in the field of optical telecommunications, e.g. for wavelength division multiplexing (WDM), for compensating chromatic dispersion in optical fibers, for stabilizing and flattening the gain of optical amplifiers, for stabilizing the frequency of semiconductor lasers, and more generally in various filters. They are also found in instrumentation, e.g. as narrowband wavelength-selective reflectors for fiber lasers, or as sensors for the measurement of strain, temperature, and hydrostatic pressure.
The Bragg wavelength depends on the period of modulation and on the average value of the refractive index in the fiber. Both quantities vary nearly linearly with the ambient temperature and the stress applied to the fiber. This, in turn, translates into a nearly linear variation of the Bragg wavelength with temperature and stress. For example, the Bragg wavelength of a typical FBG increases with temperature at a rate of about 10 pm/° C. at 1550 nm. As a result, fiber Bragg gratings are well suited for use as strain or temperature sensors. The thermal dependence represents, on the other hand, a major disadvantage for applications requiring a good stability of the spectral response of the FBG. It prevents the use of FBGs as frequency standards in advanced communication networks and in commercial systems, which typically have to operate over an extensive range of temperature.
In DWDM (Dense Wavelength Division Multiplexing) systems, for example, optical filters for adding/dropping optical signal channels must have high wavelength accuracy and high stability against changes in environmental conditions. For an allocation in their spectral domain with a spacing as narrow as 0.4 nm, the thermal variability of FBGs restrains the operational range to a few degrees, which is clearly too restrictive. Gain flattening filters (GFFs) and dispersion compensators using chirped FBGs must also have a stable spectral response to maintain their performance. For the accurate and reliable long-term operation of these devices, suitable temperature compensation techniques are a necessity.
Different temperature stabilization techniques have been proposed in the past, and there are already many known various types of packages for holding Bragg gratings constructed in such a way as to render their wavelength insensitive to temperature changes. A good survey of temperature-compensation techniques for fiber Bragg gratings is presented in S. Magne, S. Rougeault, M. Vilela, and P. Ferdinand, “State-of-strain evaluation with fiber Bragg grating rosettes: application to discrimination between strain and temperature effects in fiber sensors”, Appl. Optics, December 1997, Vol. 36, No. 36, pp. 9437-9442. They can be classified according to whether they are intrinsic i.e., make use of the fiber properties themselves, or extrinsic i.e., require an extra material. Intrinsic methods generally rely on a second grating being used in parallel for calibration purposes. These methods have been devised mostly for sensing applications, in order to compensate for the thermal sensitivity of FBG sensors used to measure physical parameters such as strain, other than temperature. For example, the resonance wavelength of a FBG sensor glued to a piece of metal to measure its expansion (or contraction) will react to the strain resulting from the thermal expansion (or contraction) of the metal, but also to the change in temperature causing this expansion (or contraction) in the first place. In order to distinguish between the two effects, a second FBG located in the vicinity of the first one but not glued to the piece of metal, is used to measure the change in temperature only. A correction to the response of the first FBG is then carried out post measurement in order to determine the expansion (or contraction) of the metal. Such calibration methods are unsuitable for telecommunications applications, where the spectral response of each individual FBG must be stabilized against temperature fluctuations, a task carried out however by extrinsic systems.
A first class of extrinsic systems relies on the active stabilization of the FBG spectral response. Certain parameters are then continuously monitored and dynamically controlled with a feedback loop. For example, active temperature control of the grating environment is typically accomplished by a stabilization system that holds the temperature at a level above the maximum ambient temperature to which the device is expected to be exposed. The temperature control can be carried out with devices such as Peltier elements. In other systems, the Bragg wavelength is monitored continuously and corrected by straining the fiber with piezoelectric elements. While being an effective approach, active thermal stabilization is costly to implement, its complexity leads to reliability concerns, and the power consumption of control circuits represents a major handicap. In general, preference is given to so-called passive devices, since they are much simpler and require no power source.
Passive temperature compensation devices generally operate by controlling the elongation with temperature of the optical fiber containing the FBG. This is usually accomplished by clamping the fiber containing the FBG to a mechanical structure that imposes a negative elongation to the fiber as the temperature increases. This contraction of the fiber compensates for the increase in its index of refraction with temperature, thus allowing a stabilization of its Bragg wavelength against temperature fluctuations.
Conceptually, the simplest method to achieve this thermal compensation is by attaching the fiber containing the FBG to a material having the desired intrinsic negative coefficient of thermal expansion The support material therefore tends to stabilize the Bragg wavelength around its initial value. Examples of such devices using certain glass-ceramics as the support material are for example described in D. L. Weidman, G. H. Beall, K. C. Chyung, G. L. Francis, R. A. Modavis, and R. M. Morena, “A novel negative expansion substrate material for athermalizing fiber Bragg gratings”, 22nd European Conference on Optical Communication—ECOC'96, Oslo, Paper MoB 3.5, pp. 1-61 . . . 63, D. Weidman, “Fiber Bragg gratings enhance real-world applications”, Laser Focus World, pp. 99-103, March 1999, U.S. Pat. No. 5,694,503 (FLEMING et al) and in U.S. Pat. No. 6,087,280 (BEALL et al).
Devices using a material with an intrinsic negative coefficient of thermal expansion suffer from major drawbacks. The coefficient of thermal expansion (CTE) must be accurately matched to the optical fiber properties, a requirement that can be met only through a careful control of the material formulation. It must also be constant from one sample of the material to another, which is difficult to achieve in practice. Such materials are difficult to machine without spoiling their properties, and in particular without altering their coefficients of thermal expansion. Also, problems may occur with the supply of these esoteric materials (like β-eucryptite glass-ceramics substrate).
Such passive temperature compensation can also be achieved through the principle of differential expansion. The fiber containing the FBG is then clamped to a structure made of materials having different, but usually positive, coefficients of thermal expansion. The structure is arranged such that the different rates of expansion between the structural elements supporting the fiber result in a negative elongation of the fiber with increasing temperature. Typically, the fiber is stretched at low temperatures and allowed to relax as the temperature increases.
Many devices that employ materials with dissimilar positive thermal expansions to achieve the required negative expansion are known. Examples of typical prior art of passive temperature-compensating packages include U.S. Pat. No. 4,936,646 (ENOCH et al), disclosing relatively temperature insensitive fiber Bragg gratings. This is one of the earliest references describing the invention of an athermal package for optical fibers. U.S. Pat. No. 5,042,898 (MOREY et al) discloses a similar idea, associating aluminium as the material having the greater coefficient of expansion with Invar, silica, stainless steel, or iron as the material having the smaller coefficient of expansion. G. W. Yoffe et al. in G. W. Yoffe, P. A. Krug, F. Ouellette, and D. Thorncraft, “Temperature-compensated optical-fiber Bragg gratings”, Optical Fiber Comm. Conf., Vol. 8, 1995 OSA Technical Digest Series, Paper W14, p. 134 and G. W. Yoffe, P. A. Krug, F. Ouellette, and D. A. Thorncraft, “Passive temperature-compensating package for optical fiber gratings”, Appl. Optics, Vol. 34, No. 30, October 1995, pp. 6859-6861 disclose one of the first practical devices for packaging Bragg gratings. These papers also describe an active adjustment to set the strain of the fiber to the desired initial value. T. E. Hammon, J. Bulman, F. Ouellette, and S. B. Poole, “A temperature compensated optical fiber Bragg grating band rejection filter and wavelength reference”, OECC '98 Technical Digest, pp. 350-351, 1996 similarly present packaging structures utilizing a combination of two materials having different thermal expansion coefficients.
A variation of the differential expansion method exists in the form of mechanical bending deformation compensation systems, such as disclosed in U.S. Pat. No. 6,044,189 (MILLER).